<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
    window.onload = function () {
        var n =prompt("请输入一个数字")
        function fn(n) {
            if (n == 1 || n == 2) {
                return 1
            };
            return fn(n - 2) + fn(n - 1);
        }
       
        
        var num=fn(n)
        console.log(num)
    }
</script>
</body>
</html>